People are defined by society by who they're dating. A man dating a woman is automatically assumed to be straight. A man dating a man is automatically assumed to be gay. Trying to challenge that and constantly tell people that you're actually bisexual is tiresome and usually pointless given how closed minded most people are, so it's obvious why people tend to give up and lose their bisexual identity. A bisexual guy who prefers women is very unlikely to come out, there's almost no positives to doing so

Exactly, I totally agree. It sounds politically incorrect, but actually I find a lot of straight people are more accepting of bisexuality. Even though they call it the LGBT community, a lot of gay people don't accept bisexuality. They think bisexuals are liars and are in denial about being gay. You aren't accepted in the gay community if you have sex with both genders. So if you come out as bisexual you won't be accepted in the traditional gay community so it doesn't work to come out like it does for a 100% gay person.